The whole point is that Meryem's strength surpasses strategy, tactics, or special abilities. Going way back in the materialization example, you can't just create aura power that affects an entity whose power is far beyond the limit of human technology. Or, put it another way, you're not going to create a damage effect more powerful than the Rose, or a special effect more devastating than the Rose poison.
I think the bit about conjured weapons being at most equivalent to real world ones is dubious. Truth is you can conjure all sorts of strange weapons and gadgets that have effects that are beyond the reach of technology. For example, how do you measure the amount of damage Sphinx does with while revved up? How much can he revv up? What sort of damage are we talking about here? It's basically impossible to compare. What about Gon's transformation condition? Whatever that condition imposes on him is not something technology could reverse or even recognize but, in the end, was cured by Nen. A certain Nen beast has the horrifying ability to fly in the air without wings and keep someone alive whilst consuming their brains.
Of course you could say that it's technologically possible - it just hasn't been discovered/invented yet. But in that case, you are effectively saying that
anything is technologically possible. If so, Nen should be the same.
The Rose isn't even the be-all-end-all of explosions anyway. It's a miniature nuke that fit in Netero's chest. Pufu waded into ground zero whilst it was still ablaze and was absolutely fine with the heat.
In fact, if anyone can do either of those power they'd already be pretty much undefeatable in the world of HXH. So if you assume Meryem is already poisoned it's pointless to assume another special power could weaken him because he's already weakened by a power that's far beyond the limit of Aura (there's no possible way you can create Rose poison with Aura).
It could be something as simple as "immobilze the target" after fulfilling condition "xyz". Or put the target into a state of Zetsu if you sacrifice a hundred baby infants. Could be anything, really. There's no limit to what Nen could do and what it personally means (which affects how strong it is) to a person.
Your specialist guy would pretty much all just die before they even see Meryem since he's just that fast. You woudln't have time to charge up anything, because Meryem would kill you before you're done charging up any attack. Even if you had an ability like Cheetu's, Meryem would immediately catch up to Cheetu in this case and then break out of the artifical dimension, so even artifical dimension can't stop him.
Or you could be like the Chameleon guy. His ability isn't all that inconceivable. Not sure if you're invisible to En but the King isn't going to be firing off En every second. I personally think his speed is over-rated. Those manga panels don't give a real perspective on the amount of time that transpires between them.
